#app4{}
#app4 .circle{
  width: 15vw;
  height: 15vw;
  border-radius: 50%;
  border: 1px solid red;
}
@keyframes change{
  0%{
    background-color: red;
  }
  100%{
    background-color: blue;
  }
}
#app4 .circle.active{
  animation: change 1s infinite alternate linear;
}